This invention relates to coil transducer motor assemblies and particularly to ironless and leakage free coil transducer motor assemblies.
This invention is disclosed in the context of a moving voice-coil transducer motor assembly for a loudspeaker. However, it is believed to be useful in other applications such as microphones, geophones, and shakers.
Voice-coil transducer motor assemblies, such as those used in traditional electrodynamic loudspeakers comprising magnetic field generating means adapted to generate a magnetic field in which a coil fixed on a moving part can be driven by a driving current in order to induce vibrations to a diaphragm connected to the moving part to produce sound, present a number of well-known drawbacks.
First, the presence of iron spacers, that usually comprise so called rear and front plates and a pole piece to help control the magnetic field characteristics in such motors leads to several kinds of nonlinearities. These include Eddy currents, the magnetic saturation of the iron and the variation of the coil inductance with its position causing a reluctant effect. However, it is desirable for the force applied on the moving part to be an image of the driving current. The driving forces applied on the moving part of the loudspeaker can be written as follows:
                              F          driv                =                                            F              L                        +                          F              r                                =                      Bli            +                                          1                2                            ⁢                                                ⅆ                  L                                                  ⅆ                  x                                            ⁢                              i                2                                                                        Equation        ⁢                                  ⁢                  (          1          )                    
Where FL is the Laplace force, Fr the reluctant force, B the induction seen by the voice-coil, l the length of the coil, i the driving current flowing through the coil, L the inductance of the coil and x the displacement of the coil. Thus, equation (1) shows that if the inductance of the coil varies, a reluctant force, proportional to i2, occurs and interferes with the Laplace force. This reluctant force creates a force distortion resulting directly in an audible acoustical distortion.
Second, a significant part of the magnetic field created by most loudspeaker motors does not contribute towards making the diaphragm move. In addition to a simple loss of magnetic field, this leakage flux can be attracted by any ferromagnetic object placed nearby, leading to a decrease of the device efficiency. Reciprocally, this leakage magnetic field can prevent some devices placed nearby from working properly.
In order to solve these problems, several structures of ironless coil transducer motor assemblies have been proposed, one example of which is disclosed in the patent document FR2892886.
This disclosed assembly comprises a plurality of sintered permanent magnets arranged in such a way that the magnetization is always parallel to the outer edge. The perpendicular arrangement of the magnets leads to the generation of a magnetic field by the motor that is focused on the coil path without the use of iron spacers to focus and guide the magnetic field. The inductance of the coil no longer depends on its position, resulting in the vanishing of the reluctant force and the other nonlinearities due to iron that were listed previously. In addition, the inductance is diminished and consequently, so is the electrical impedance, especially at high frequencies.
However, although some field leakage is prevented in comparison with a traditional coil transducer motor assemblies comprising iron spacers, it is still a drawback that these assemblies have magnetic field leakage especially towards the external parts of the assembly, that prevent integration of such assemblies in close neighborhood of other electrical devices.
Another problem of this ironless coil transducer motor assembly is that the structure made of sintered magnets is difficult to assemble, as it requires the manufacture of magnet rings with distinct magnetization directions especially for the radially magnetized magnet rings and to have them sintered together.
These two problems are emphasized the more the dimensions of the loudspeaker are reduced.